1. Field of the Invention
This invention relates generally to a method of preparing for structural analysis a deep trench (DT)-type capacitor, and a method of structural analysis therefor. The invention relates more specifically to a method of preparation which employs a combination of mechanical and chemical action to expose the trench-type capacitors for structural analysis.
2. Description of Related Art
Demands for miniaturization in the semiconductor industry require that a higher density of electronic devices such as capacitors be placed on a chip.
One solution to the demand for higher density capacitors is the use of trench-type capacitors. In a trench-type capacitor, advantage is taken of the depth of the substrate in order to place a high density pattern of capacitors on the semiconductor device. For example, U.S. Pat. No. 6,071,823 describes a method of fabricating a bottle-shaped deep trench in a semiconductor substrate.
For various reasons, different types of structural defects can occur during fabrication of the semiconductor. These include such defects as metal shorts, capacitor holes, particle failures, incomplete deep trench formation and deformation.
Because the semiconductors have become so miniaturized, however, the ability to detect such structural defects has become more challenging.
One conventional method of preparing a deep trench capacitor for structural analysis for such possible defects is described in U.S. Pat. No. 5,064,498, which is incorporated by reference herein in its entirety. In this method, the silicon substrate of the chip is etched away, leaving the memory cells exposed for viewing. The method includes the steps of removing oxide from the backside of a semiconductor device, and placing the semiconductor device into a solution of choline and water. The solution etches away the substrate. The memory cells may be photographed and viewed by TEM and SEM techniques.
However, in this prior art process, it is difficult to control the chemical etching of the substrate to remove a precise thickness thereof; thus, the risk of removing the deep trench capacitor is great. Also, the chemical etching requires a long time to effect. Further, the reaction of the choline/water solution is slow and unstable, and it is difficult to determine the stop point.
Therefore, a need has existed for a technique which overcomes the above-described deficiencies associated with conventional methods.
It is an object of the present invention to provide for an efficient and reliable method of preparing a large area deep trench capacitor for three-dimensional analysis by SEM or optical microscopy, or lift-off from the die and removal to a carbon film grid for TEM observation.
More specifically, the present invention is directed to a method of preparing for structural analysis a deep trench-type capacitor formed in a semiconductor die, employing a combination of mechanical and chemical action to expose the trench-type capacitors. The method of preparing the die includes the steps of (a) mechanically treating the substrate disposed at the die back side so as to remove a first portion of the substrate and leave intact a second portion of the substrate; (b) mounting the mechanically treated die by affixing the die by its top side to a mount; and (c) chemically treating the die so as to remove the substrate second portion. By exposing the deep trench capacitors, the method facilitates the inspection of the device for the detection of possible structural defects such as metal shorts, capacitor holes, and particle failures. The method further overcomes the deficiencies associated with conventional methods of substrate removal, and facilitates inspection by a variety of methods. The method facilitates, for example, the detection of residual arsenic glass, and deep trench deformities, such as those associated with deep trench length and the thickness of the deep trench ON layer.
According to one aspect of the present invention, there is provided a method of preparing a deep trench-type capacitor for analysis, the deep trench capacitor being formed in a substrate of a semiconductor die comprising a top side and a back side, the method comprising the steps of (a) mechanically treating (for example, by polishing for a period of approximately 30 minutes with a diamond film) the back side of the die so as to remove a first portion of the substrate; (b) mounting the mechanically treated die by affixing the top side of the die to a mount (formed of for example silicon or glass construction); and (c) chemically treating the die so as to remove a second portion of the substrate to expose the at least one deep trench capacitor for three-dimensional observation.
The first portion of the substrate removed in step (a) may be a layer having a thickness of from 20 to 100 xcexcm, and preferably a thickness of 60 xcexcm. Step (b) may comprise affixing the top side of the die to the mount with an adhesive, such as an epoxy that is then allowed to dry for a period of from 30 to 120 minutes before proceeding to step (c).
Step (c) may comprise chemically treating by exposing the second portion of the substrate to a chemical solution, which may include organic and/or inorganic chemicals. The chemical solution may be a 6-50 wt % solution of KOH, and preferably a 16 wt % solution of KOH, having a temperature of from 25 to 80xc2x0 C., for treating for a period of approximately 60 to 600 minutes. Step (c) may alternatively include chemically treating by exposing the second portion of the substrate to reactive ion etching (RIE) or by exposing the second portion of the substrate to plasma etching.
The method may further comprise after step (c) further steps including rinsing the chemically treated die for example with methyl alcohol or ethyl alcohol and drying the rinsed die for example at a temperature of from 100 to 150xc2x0 C. for a period of from 3 to 5 seconds. The method may further comprise after step (c), a further step of removing the DT capacitor from the chemically treated die and affixing the removed DT device to a carbon film grid for TEM observation.
According to another aspect of the present invention, there is provided a method of analyzing the structure of a deep trench-type capacitor, the deep trench capacitor being formed in a substrate of a semiconductor die comprising a top side and a back side, the method comprising the steps of (a) mechanically treating the back side of the die so as to remove a first portion of the substrate; (b) mounting the mechanically treated die by affixing the top side of the die to a mount; (c) chemically treating the die so as to remove a second portion of the substrate to expose the at least one deep trench capacitor for three-dimensional observation; and (d) performing a structural analysis of the at least one deep trench capacitor. Step (d) may include performing the structural analysis by a method selected from the group consisting of scanning electron microscopy and optical microscopy. The method may further include after step (c) and before step (d), a step of removing the DT capacitor from the chemically treated die and affixing the removed DT device to a carbon film grid for TEM observation. Step (d) may comprise performing the structural analysis by transmission electron microscopy.
According to another aspect of the present invention, there is provided a method of preparing for structural analysis a semiconductor device comprising at least one capacitor disposed in a substrate of a die, the method comprising chemically treating the substrate so as to remove the substrate by one of (i) exposing the substrate to a solution of KOH, (ii) reactive ion etching, and (iii) plasma etching, in order to expose the at least one capacitor for structural analysis. This method may further comprise, before the step of chemically treating, a step of mechanically treating the substrate so as to remove a first portion thereof and removing the at least one capacitor from the chemically treated die and affixing the removed DT device to a carbon film grid for TEM observation.
The present invention, therefore, provides a method of preparation which advantageously overcomes the deficiencies associated with conventional methods, and which is advantageously suitable for any service in which a simple and fast method of preparation is desired. The method facilitates, for example, the detection of residual arsenic glass, and deep trench deformities, such as those associated with deep trench length and the thickness of the deep trench ON layer.
Also, compared to the above-discussed technique of U.S. Pat. No. 5,064,498, the mechanical treatment to remove the first portion of the substrate according to the present invention facilitates more precise control of the thickness of the removed portion of the substrate and is quicker. Further, while the reaction of a choline/water solution is slow and unstable, with difficult determination of the stop point, the RON solution employed in the present invention is a common and safe solution.